Hi Swallowtail
I bought one of these and installed it myself. It uses low power FM to transmit the decoded DAB+ signal to the RNS850 and it uses the RDS function to display the station you have selected (via the supplied remote control) on the RNS screen. You don't get details of song playing etc. But you DO get a choice of all DAB+ stations wherever you are including the ones without any constant nagging ads and blah blah blah!! Meets my criteria!!
It uses a supplied powered DAB antenna which I mounted at the top of the passenger side of the windscreen. If you smash the windscreen, you'd be looking for a new antenna as it is WELL stuck on. I mounted the decoder in the LHS of the dash behind the fuse box and ran 12V from a switched 12V rail (back of a 12V socket) so the unit only turns on when the ignition is on. No risk of battery drain. If I want to turn the radio on when the car isn't on, then I just select one of the other FM stations.
http://www.axionag.de/products/axion/dab210vw-dab-universelle-dab-digitalradio-nachruest-box_en
I bought it through Amazon in Germany. It is also sold as an OEM VW accessory in some parts of the world. My carton was badged VW when it arrived in Aus.
It works well from over 12 months experience.
That might provide you with another potential solution - not integrated like you were trying, but also at about a third of the cost.
Cheers
Andrew